The present invention relates to pumps, and more particularly to pump apparatus and methods for pumping molten metal.
The use of pumps to pump molten metal such as aluminum or zinc is known in the art. Generally, molten metal pumps comprise centrifugal pumps modified to provide processing of the molten metal. To that end, circulation pumps are used to equalize temperature and improve homogeneity of mixture in a molten metal bath, transfer pumps are used to convey or transfer molten metal between locations and gas-injection pumps are used to circulate and inject gas into a molten metal to modify its composition as by removing dissolved gases or dissolved contaminant metals therefrom.
The pumps typically include a base or casing having a pumping chamber and an impeller received within the chamber. The base includes inlet and outlet passages for intake and discharge of the molten metal being pumped. The pump may be a volute pump wherein the pumping chamber has a volute shape comprising a spiral configuration of circumferentially increasing cross sectional area approaching the pump outlet passage. It is also possible to provide the pump with a pumping chamber having a generally circular shape.
The pump base together with the impeller are submerged in the molten metal and connected via a plurality of support posts to a drive arrangement positioned above the level of the molten metal. The impeller is supported for rotation within the pumping chamber by a rotatable shaft coupled to the drive arrangement. In typical installations, the drive shaft may be of various lengths, e.g. one to four feet in length or longer, in order to provide adequate clearance above the molten metal level.
A typical impeller includes at least two axially extending vanes and a radially extending member which forms a base when located below the vanes. In this manner the impeller provides a vane array with adjacent vanes cooperating with the base to form vane pockets. During pumping, molten metal is axially introduced into the pockets and laterally ejected due to centrifugal force.
The necessary spacing between the driver and impeller results in the use of an elongate drive shaft fixed to the impeller. This requires a relatively high degree of balance during operation and adequate bearing support between the impeller/shaft assembly and the housing. Operating vibration may damage the pump and/or limit its pumping efficiency.
The impeller may be fractured or otherwise damaged due to the vibrations and failure to maintain operating clearances. In molten metal pumping systems, bearings may be considered to operate on films of molten metal and poor concentricity yields reduced clearances which may cause the films to break down or not form so as to give rise to refractory material wear of increased rate.
The pumps and methods are characterized by unique fluid flow properties tending to smooth the rotation of the impeller by better equalizing the pressure between each pair of vanes within the vane array. This tends to reduce pump damage and bearing wear by suppressing repeated vibrational impacts during pump operation, e.g., chatter, while providing improved pump performance.
These improvements are achieved in part by the provision of circumferential feed flows of molten metal to the interior regions of the impeller during pumping. The circumferential flows are provided through openings extending through the vanes. The circumferential flows tend to enhance the completeness and uniformity of the filling and evacuation of the vane pocket between each pair of vanes by accelerating a flow of metal into a lower region of the pocket.
The advantages of circumferential feeds to the vane pockets or interior regions of the vane array of the impeller appear to relate to the rapid input of metal to the vanes pockets following the pumping radial ejection of metal therefrom. As the impeller begins its uniform circulatory motion, the continuity of the filling and emptying of the vane pockets with molten metal is enhanced by the circumferential flows through the vanes in accordance with the invention. The quicker one can get the media or molten metal to occupy that empty space the quicker the media will pump.
The fluid flow properties are further enhanced by the improved balancing or equalization of pressure within the vane pockets which are believed to reduce vibrations and fluid flow irregularities during pumping. In turn, the smoothness of impeller rotation tends to be enhanced by the increased continuity of the pumping action.
The openings extend from the opposed surfaces of the vane. The openings may be disposed at any location extending through the circumferential or peripheral thickness of the vanes and may have any desired axial or radial orientation. Thus, the openings may be inclined upwardly or downwardly relative to the direction of impeller rotation or in an orientation generally parallel with the impeller base.
The fluid or molten metal flow through the opening in the vane of the impeller is enhanced by disposing the opening at an inclined angle. That is, the opening is inclined upwardly into the direction of rotation of the vane so that an intake vector force is imposed on the fluid to bias flow into the opening and the interior region of the impeller. The angular orientation of the opening imposes an intake vector force on the molten metal that operates to expedite metal flow into and through the opening.
At least one opening may be provided in at least one vane. More preferably, a single opening may be provided in each vane or in less than all vanes provided a majority of the vanes include at least one opening. Accordingly, the impeller may include an imperforate vane.
Multiple openings may be provided in one or more of the vanes. Thus, an impeller may include imperforate, single opening and multiple opening vane or vanes. The rotational balance of the impeller and/or suppression of chatter characterized by repeated or regular vibrations may be reduced by trial and error depending upon the interaction of the impeller configuration, radial member or pump base and bearing mounting system.
The opening may have any convenient cross-sectional shape. For example, a circular cross-section is convenient, but oval or other shapes may be used. Further, the shape and/or size of the cross-sectional opening may vary along the axial length of the opening. For example, an opening may be provided with an enlarged inlet to enhance fluid intake.
In a further aspect of the invention, at least one drain hole provides safe drainage of molten metal from the impeller during removal of the pump from the molten metal for service or the like. The drain hole may extend through the radial member or base of the pump and be located in one of the vane pockets.
The single drain hole also tends to prevent thermal shock as the pump, or more particularly the impeller, is submerged into the molten metal. Following service of the pump apparatus, the impeller is relatively cool. As the impeller is submerged into the molten metal, the lower extremities of the impeller or impeller base are rapidly heated. Such rapid heating from a single side of the impeller raises the possibility of thermal shock and fracture of the refractory cement mounting the bearing ring to the impeller base. Accordingly, the rapid flow of the molten metal through the drain hole to upper impeller locations or top surface of the base tends to uniformly heat spaced regions on the impeller so as to suppress the possibility of thermal shock and fracture of the refractory cement.
Impeller drainage may be further improved by connecting the vane pocket in which the drain hole is located to other vane pockets by openings extending through the vanes. In such an arrangement, the advantages of circumferential flow and pressure equalization of the vane pockets are also achieved.
The selective angular placement of the drain opening or hole also serves to better balance the impeller. For example, the impeller may be characterized by material, configuration or dimensional variations which detract from true or balanced rotation without vibration. These variations may be offset by placement of the drain opening adjacent a location of increased angular momentum or higher rotational weight or the like that tends to detract from smooth rotation.
In accordance with yet another aspect of the invention, one or more additional hub drain holes may be provided. Such hub drain holes comprise openings extending through the impeller hub or other structure located just above the impeller radial member or base and communicating with the impeller drive shaft opening. As indicated, such hub drain holes are positioned just above the impeller radial member or base in order to enhance complete drainage of the vane pockets.
In accordance with a further aspect of the invention, an improved impeller includes a body having a longitudinal axis and a plurality of elongate pumping chambers located adjacent the peripheral extremities of the body. The impeller body includes an end surface and a peripheral surface. The pumping chambers comprise elongate cavities or bores that intersect the end surface of the body to form cooperating impeller inlet openings and the peripheral surface of the body to form cooperating impeller outlet openings.
The pumping chambers have a length and a transverse width. The length to width ratio is 3:1 or greater, and more preferably, is in the range of from about 3:1 to about 20:1, and more preferably, from about 3:1 to about 5:1.
In illustrated embodiments, the impeller body has a cylindrical shape and each pumping chamber has a length that extends in a linear direction along the peripheral or cylindrical surface of the body. The pumping chambers extend along 10 to 100% of the longitudinal dimension of the body, or more preferably from 20% to 85%.
The pumping chamber may be disposed at an angle with respect to the longitudinal axis of the body ranging from 0xc2x0 to 45xc2x0. The pumping chambers are inclined into the direction of impeller rotation and provide multiple flow pumping forces. More particularly, the inclined pumping chambers provide axial pumping by applying an axial force vector to the fluid as well as radial pumping by applying centrifugal force to the fluid in the chamber. Such multiflow pumping yields increased pressure and flow as compared with similarly sized impellers not having axial pumping.
As indicated, the pumping chambers are located adjacent the radial extremities of the body. Preferably, the pumping chambers are located in the outermost ⅓ of the transverse or radial dimension of the body.